Electrical prospecting system



May 15, 1945.

Izumi/vr @NI d ELECTRICAL PROSPEGTING SYSTEM H. M. EVJEN Filed April 29, 1943 l -l al Il Il ik l IN VEN TOR.

fau/fan 1% E11/'en /7 TTRNE Y Patented May 15,1945A ELECTRICAL PROSPECTING SYSTEM Haakon M. Evjen, Houston, Tex.,"assignor to Nordel Corporation,`Houston, Tex., a corporation of Delaware Application April 29. 1943, serial No. 484,990

claims. (ci. lis-'182) This invention relates lto -systems for making geo-physical explorations wherein a current having a controlled low frequency of the order of from one-half vcycle to 20 cycles per second is passed through the earth `between spaced electrodesand the groundfpotential is' picked up by.

a pair of potential electrodes located within the field of influence of the earth current;

I have now discovered that when a commutated current is passed into the ground Vthe po tential difference between any two fixed points in .the ground will generally not be in phase with A phase displacement will exist the current. whichfvaries with the .frequency of alternation or commutation Vof the current. From the manner in which this phase displacement varies with frequency useful deductions can be drawnconcerning the presence of valuable minerals in the ground. Such measurements are particularly significant atV very low frequencies Awhere the ordinary alternating current technique will `not produce satisfactory results.

The present invention provides a system for driven switch to provide an alternating current" in the form of spaced flat-toppedpulses.

A current pulse of definite duration is passed into `the ground in one direction. The current is then interrupted for a definite period of time, hereinafter called the current gap, and is then passed into the ground in the opposite direction, after which the current is again interrupted and the cycle repeated. For convenience and, by analogy to usual alternating current terminology,

the tangent of the phase angle is defined as the ratio of the time average of the potential drop between given points during a current gap interval to the time average of the potential drop between said points while thecurrent is passed through the current electrodes into the ground or, expressed in another'way, the ratio between the `out-offphase picked-up potential to the inphase picked-up potential.

. It is an object of the present invention to provide aA novel and improved system for measuring theabove mentioned ratio.

' gap intervals.

Another object is to provide al geophysical prospecting system having novel and improved characteristics.

Various other objects and advantageswill be Vapparent as the nature of the invention is more fully described.

The invention will be better understood by referring to the following description, taken in connection with the accompanying drawing in which va specific embodiment thereof has been set forth for purposes of illustration.

In the drawing,

Fig. 1 is a diagrammatic representation of a geophysical prospecting system embodying the present invention; and

Fig. 2 is a series of curves illustrating the operation of the system shown in Fig. l.

In the following description certain specific terms have been used for convenience` in referring to various details of the invention. It is to be understood, however, that thesefterms are to be given as broad an interpretation as the state of the art will permit.

Referring to the drawing more in detail, a source Il of direct current is connected rby means of lines and l2 to movable contacts I3 and I4 respectively of a motordriven reversing switch I5, which may be of any suitable type, for example, of the type set forth in Evjen Patent 2,314,874, dated March 30, 1943, which shows a switch adapted to reverse the circuit connections at a predetermined frequency and having an adjustable gap or clearance to control the times of reversal, the current intervals, and the The switch may be driven by suitable means indicated diagrammatically at I1. An ammeter |6 is shown as connected in the line I2 for purposes of regulation and control.

The reversing switch |5 is shown as-provded with a pair of stationary contacts 20 connected by a line 2| to an earth current electrode 22, and with a pair of stationary contacts 23 connected by a line 24 to a second earth current electrode The potential pick-up circuit comprises a pair of pick-up electrodes 30 and 3| which are located withn the field of influence of the earth current. The pick-up electrode 3U is connected ,by a line 32 to stationary contacts 33 of a second reversing switch 34 which is similar to the switch I5 above referred to and is driven by driving mechanism indicated at 29. The pick-up electrode 3| is connected by a line 35 to stationary contacts 36 of said switch 34. The movable.

contacts 31 of said switch 34 are connected by lines 33 to a direct current measuring instrument 33 shown as a null balance galvanometer.

'The measuring circuit comprises a condenser 43 connected to the movable contacts 4i of a re versing switch 42 which is similar to the reversing switches I5 and 34 above described and is driven by suitable mechanism indicated at 49. Stationary contacts 43 and 44 of the reversing switch 42 are connected by lines 4l and 48 to lines 35 and 32 respectively. Stationary contacts 41 and 48 of the said reversing switch 42 are connected in a series circuit comprising potentiometer 50 and impedance 5I. The potentiometer I3 is provided with adjustable contacts 62 which are connected to the blades of a manual reversing switch I3. The switch 53 is provided with stationary contacts 54 which are connected in series with the line 35.

'I'he reversing switches l5 and 34 are intercon.

nested-for operation in unison. but 90 out of phase,fso that the contacts of one switch are closed while the contacts of the other switch are open and vice versa. The reversing switch 42 is connected to be driven at double the speed of the switches I5 and 34 and is so' adjusted that the contacts 41 and 48 are closed when the contacts of the reversing switch 34 are closed, and the contacts 43 and '44 are closed when the contacts of the reversing switch l 5 are closed.

The sequence of operation of the reversing j switches is illustrated in curves A, B and C in Fig. 2. Curve A represents the current passed into the ground from electrodes 22 and 25 and it will be noted that the current is passed through the ground in one direction during a period of 60, is interrupted for an interval of 120 and is then passed through the ground in the opposite direction for a period of 60.

The curve B in Fig. 2 indicates the operation of the reversing switch 34, which is set for 60 measuring intervals and 120 gap intervals. The current intervals are offset, however, to occur at the center of the gap intervals of curve A and vice versa, so that the potential pick-up circuit may be considered to be operated 90 electrical degrees out of phase with respect to the earch current circuit. I

The operation` of the reversing switch 42 is indicated -by curve C in Fig. 2. It will be noted v"that the contacts 43 and 44 are closed to connect the condenser 40 directly across the potential pick-up electrodes 30 and 3| during the 60 current period oi' the earth current circuit. Hence,

the condenser 40 is thus charged to a potential proportional to the amount of in-phase potential which is picked up by the electrodes 30 and 3|. The contacts 41 and 48 of the reversing switch 42 are closed, however, during the 60 period that the reversing switch 34 is closed. During this period the condenser 40 discharges through the circuit including the potentiometer and the impedance 5| to produce a potential drop across the potentiometer 50 which is proportional to the in-phase picked-up charging potential. An adjustable portion of this potential drop between the contacts 52 is applied to the measuring circuit to oppose the out-of-phase -potential picked up by the pick-up electrodes 30 and 3|. The direction of this potential ldrop may be reversed if necessary by the manual reversing switch 53 and the contacts 52 are adjusted to obtain a null balance of the galvanometer 39. When this balance is effected, the position of the contacts 52 is a direct indication of the ratio :between the inphase picked-up potential and the out-of-phase picked-up potential, which corresponds to the phase angle.

These measurements are repeated at different frequencies by varying the speed of operation of reversing switches I5, 34 and 42 and a. log o1' the variation of phase angle with frequency is thus obtained.

It has been found that the presence of oil and other valuable minerals in the ground causes significant anomalies in the phase angle. 'I'he behavior of the phase angle with variations in frequency can thus be used as a direct indication of the presence of these materials.

The reversing switch 34 serves as a rectifier to rectify the potential impulses which are-picked up by the Vpick-up electrodes 34 and 3| and thus permits the use of a direct current measuring instrument, such as the galvanometer 39. f

Any residual earth current which would tend to cause fluctuation of the galvanometer 33 may be balanced out if desired by a suitable adjustable source of direct potential which may be connected 1n the potential pick-up circuit on the electrode side of the switch 34, forexample a variable resistor 30 connected across a battery Il the midpoint of which is connected to' theyelectrode 34. Such potential, if-used, may beso adjusted as to eliminate fluctuation in the` galvanometer 33. If

desired, the switch 34 may be adjusted to short circuit the galvanometer 33 during potential gap intervals so as to prevent potential surges when the reversing contacts close.

Although certain specific embodiments of the invention have been shown for purposes of illustration, itis understood that the invention is not to Ibe restricted thereto but is capable of various uses as will be readily apparent to a person skilled in the art. The invention is to be limited only in accordance with the scope of the following claims.

What is claimed is:

1. A system for electrical prospecting comprising an earth current circuit including a source of direct current, a'pair of spaced earth electrodes and a motor driven reversing switch connected between said source and said electrodes for interrupting and reversing the earth current at a pre-- determined controlled low frequency with definite adjustable gap periods at each interruption, a potential circuit including potential pick-up electrodes located at spaced points lying within the field of inuence of said earth current, a D. C. potential indicating device connected to said pick-up electrodes to indicate the potential impressed thereby on said potential circuit, a reversing switch driven in unison with said first reversing switch and connected in said potential circuit between said pick-up electrodes and said indicating device, said last reversing switch being displaced in phase with respect to said first reversing switch for closing said potential circuit only when said first switch is in open 'position and vice versa and being timed to convert the potential picked-up during saidclosed circuit periods into a uni-directional potential suited to actuate said indicating device, a potentiometer, a condenser, a third reversing switch driven in unison with said first two reversing switches but operating at double the frequency thereof, said third reversing switch having connections to connect said condenser across said potential pick-upv electrodes for charging and other connections to connect said condenser acrosssaid potentiometer for discharging to produce a potential drop in said potentiometer proportional to the charging potential, said last reversing switch being timed to connect said condenser across said .pick-up electrodes during earth current periods and to conneet said condenser across said potentiometer during potential measuring periods, and connections connecting a selected portion of said potentio'meter in said potential circuit in a direction `such that the potential drop therein opposes the the iield of influence of said earth current, a

D. C. potential indicating device connected to said pressed thereby on said potential circuit, a reversing switch driven in unison with said first reversing switch and connected in said potential circuit between said pick-up electrodes and said indicating device, said last reversing switch being displaced in phase with 'respect to said rst .pick-up electrodes to indicate the potential imreversing switchfor closing said potential circuit l only whenvsaid first switch is in open position and vice versa and being timed to convert the po tential picked up during said closed circuit periods into a uni-directional potential suited to actuate said indicating device, a potentiometer, a condenser, a third reversing switch driven in unison with said nrst two reversing switches but operating at double the frequency thereof, said third reversing switch having connections to connect said condenser across said potential pick-up electrodes for charging and other connections to connect said condenser across said potentiometer for dischargingto produce a potential drop in said potentiometer proportional to the charging potential, said last reversing switch being timed to connect said condenser across said pick-up electrodes during earth current periods and to connect said condenser across said potentiometer during potential measuring periods,'connections connecting a selected portion of said potentiometer in said potential circuit in a direction such that the potential drop therein opposes the said measuring instrument across said pick-up electrodes only during current gap periods, a resistor, a condenser, timed switch means to connect said condenser alternately across said pickup electrodes for charging during earth current. periods and across said resistor for discharge during said' current gap periods to develop in said resistor a potential drop proportional to the potential picked-up during said earth current periods, means applying a selected portion of said potential drop to said'measuring instrument in a direction to oppose the picked-up potential ap plied thereto during said current gap periods, and means adjusting said selected portion to balance said last potential.

4. A system for electrical prospecting comprising an earth current circuit including a source of direct current, spaced earth current electrodes and connections supplying current from said source to said electrodes for passing a current through the earth, means in said circuit to `in terrupt and reverse said current at a controlled low frequency'with controlled current gap periods at each interruption during which no current f flows, a potential circuit including potential pickup electrodes located at spaced points within the iield of influence of said earth'current, and a measuring circuit connected to said pick-up electrodes, said measuring circuit comprising a D. C. potential measuring instrument, interrupting and reversing means timed with said rst interrupting and reversing means to connect said measuring instrument `across said pick-up electrodes i only during current gap periods and to reverse picked-up potential and means adjusting said selected portion to balance said picked-.11p potential and obtain a null balance of said D. @indicating device at which point the setting of said potenl tiometer is a measure of the phase angle ofthe picked-up potential with respect to the earth current.

3. A system for electrical prospecting comprising an earth current circuit including a source of direct current, spaced earth current electrodes and connections supplying current from said source to said electrodes for passing a current through the earth, means in said circuit to interrupt and reverse said current at a controlled low frequency with controlled current gap periods at each interruption during which no current flows, a potential circuit including potential pick-up electrodes located at spaced points within the field of influence of said earth current, and a measuring circuit connected to said pick-up electrodes, said measuring circuit comprising a potential measuring instrument, switch means timed with .said interrupting and reversing means to connect the connections to said measuring instrument at successive current gap periods to rectify the picked-up potential induced by said earth current, a potential responsive circuit, means timed with said iirst interrupting means to connect said last circuit to said pick-up electrodes during successive earth current ilow periods, means in said last circuit to derive a potential proportion to the potential picked up during said successive earth current periods, and means timed with said second interruptingl and reversing means to apply said derived potential to said measuring instrument in 4a direction and at a time to oppose the picked-up potential applied thereto from said pick-up electrodes during said current gap periods to afford thereby a highly sensitive indication of their relative values. A

5. The method of electrical prospecting which comprises` passing through the earth a current in the form of a series of similar pulses, successive pulses being in opposite directions and being spaced by" a gap period during which no current ows, said gap periods and said pulse periods tog ther forming a repeated pattern of controlled frequency, picking up the potential between spaced points within the field of inuence of said earth current during the current pulse periods,

` obtaining therefrom a measuring potential proportional to the picked-up* potential, picking up during the currentgap periods the alternating 'por tential induced between said points by the earth current, periodically reversing the last picked-up potential at said controlled frequency to produce a unidirectional potential, supplying the unidirectional potential to a direct current measuring` device and balancing said measuring potential against the potential pickedup during the current gap periods to produce a null vreading on said measuring device.

HAAKON M. EVJE'N. 

